Lunar Dial

A subsidiary dial, usually situated in the arch but sometimes found as a segmental aperture in the main part of the dial, or as a circular aperture known as a 'halfpenny moon', or as the 'Halifax moon'. The complete lunar cycle takes almost 29 1/2 days, and the purpose of the lunar dial was to record automatically the lunar phases. Although a lunar dial has often been regarded simply as an interesting addition to the dial, it was of great practical use in the days before street lighting.

( Understanding the Lunar Month )